In a thread manufacturing process such as that of a screw, bolt or of a threaded pipe it is necessary to verify that the geometrical dimensions of the piece comply with the tolerances set for the product. In addition, knowledge of the nature of the deviation from these tolerances can be used for feedback to the manufacturing process, so avoiding rejects later on in the process.
A main problem in performing the measurement operations for quality control is the precision and repeatability of measurements. In the past there have been attempts to improve accuracy and repeatability of the measurement operations and to make measurement systems capable of measuring the thread shape of complex mechanical objects like the threading of pipes used in the oil industry. In this particular technical field there is the need to measure several parameters like taper of pin and box, thread pitch, thread height, pin or box diameter, pipe ovality, run in and run out.
For instance document U.S. Pat. No. 5,712,706 discloses a non-contact laser-based sensor guided by a precision mechanical system which scans a thread profile and produces a set of computer images of the threading. The computer images are then analyzed to acquire quantitative information about thread characteristics such as pitch, lead, root radius, flank angle, surface roughness, helix variation, and pitch diameter. That document however has the disadvantage that it does not address explicitly the important problem of piece misalignment and therefore it requires an absolute precision of the operations when aligning the piece to be measured with the mechanical system coordinates. This alignment can only be achieved when the piece is at the threading machine. Measuring at the threading machine has several disadvantages; it adds costly time to the threading process by preventing the inspection and manufacturing process from running in parallel, it requires placing delicate optics and precise mechanical components in a hostile environment with cutting oil and strong vibrations present, and to some extent uses the same mechanical movement that has to be verified. Once the piece has been removed from the lathe, this alignment is very difficult to achieve manually and consequently the system disclosed by that document only allows measurement of relative or local magnitudes, i.e. thread height by comparing contiguous crests and roots, whereas errors introduced by a piece misalignment are not “noticed” by that solution and in these cases they produce an insufficiently precise measurement. It also does not address the measurement of important thread parameters such as taper, run-in, run-out, black crest, length of complete thread or specific process parameters such as taper profile, pitch linearity, Fourier mode decomposition of ovality, lathe plate misalignment, hook end angle severity to name just a few.
The need is felt to solve the problem of overcoming misalignment between the measurement device and the threaded piece to be measured in a repeatable, satisfactory and sufficiently precise manner.